World's coral reefs face new peril from beneath within decades

The world's coral reefs, already enduring multiple threats from bleaching to nutrient run-off from farming, also face another challenge - this time from below.

New research, published in the journal Science on Friday, has found the sediments on which many reefs are built are 10 times more sensitive to the acidifying oceans than the living corals themselves. Some reef bases are already dissolving.

Not loving it enough: coral reefs face multiple threats from climate change, including as it turns out, from below.

The study used underwater chambers at four sites in the Pacific and Atlantic oceans, including Heron Island in the Great Barrier Reef, and applied modelling to extrapolate results for 22 reefs in three ocean basins.

As oceans turn more acidic, the corals themselves produce less of the calcium carbonate that forms their base. Instead of growing, the reef bases start to dissolve.

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"The public is less aware of the threat of ocean acidification [than warming waters]," said Bradley Eyre, a professor of biogeochemistry at the Southern Cross University and the paper's lead author.

“Coral reef sediments around the world will trend towards dissolving when seawater reaches a tipping point in acidity - which is likely to occur well before the end of the century,” he said.

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At risk will be coral reef ecosystems that support tourism, fisheries and the many other human activities, he said.

“It is vital that we put pressure on governments globally to act in concert to lower carbon dioxide emissions as this is the only way we can stop the oceans acidifying and dissolving our reefs,” Professor Eyre said.

Rates of dissolving reef sediment will depend on their starting points, including their exposure to organic sediment. The Hawaiian reef studied is already showing signs of its sediment dissolving, with higher organic nutrient levels likely to be contributing, he said.

"Carbonate sediments in Hawaii are already net dissolving and will be strongly net dissolving by the end of the century," the paper said.

Living corals themselves appear to be able to resist the acidification process, with mechanisms and strategies to resist some of the impacts.

Still, the study said the transition of the dissolution of reef sediment "will result in the loss of material for building shallow reef habitats such as reef flats and lagoons, and associated coral cays". It is unknown if the reefs will face "catastrophic destruction" once the erosion begins, the paper said.

Over time, as coral bases begin to dissolve, they are more likely to become more vulnerable to cyclones and other threats, Professor Eyre said.

He said further study was needed to understand how reefs would be affected by temperatures, rising organic and nutrient levels and more acidic waters in combination, he said.

The impact of bleaching - such as the two mass events in the 2015-16 and 2016-17 summers on the Great Barrier Reef - would most likely accelerate the breakdown of reefs by "making more sediment and organic matter available for dissolution", the paper said.

Coral reefs provide important habitats for many species, such this reef at Tufi in Papua New Guinea.